1. Field of the Invention
The present invention relates to a chiral agent for liquid crystal, which has optical activity and causes a change in a structure of the liquid crystal; a liquid crystal composition containing the chiral agent for liquid crystal, and a polymer thereof; a filter for optical recording media, which can highly prevent the occurrence of noises; a hologram type optical recording medium capable of high-density recording using the filter for optical recording media; and a color filter for liquid crystal having high color purity.
2. Description of the Related Art
Cholesteric phase is one in which molecules of liquid crystal compounds are arranged in a helical structure, and can be formed by, for example, doping a nematic liquid crystal with a chiral compound. The liquid crystal in the cholesteric phase often shows chromatic interference effect through selective reflection of light having a fixed wavelength, due to the helical liquid crystal structure. The cholesteric phase having color rendering capability can be fixed by, for example, cooling the material to a temperature lower than the glass transition point or mixing the material into a network structure of polymer, thereby making it possible to use the material as an optical filter for a specific range of wavelengths, a colored coating agent, an interfering pigment or an optical film.
The wavelength at which the cholesteric liquid crystal phase formed from a nematic liquid crystal and a chiral compound demonstrates selective reflection can be set as required by controlling a liquid crystal twisting power of the chiral agent (HTP) and the amount of the chiral agent added. As such chiral agent, various compounds are disclosed (see, for example, German Patent Application No. 4342280 and Japanese Patent Application Laid-Open (JP-A) No. 2002-180051).
A proper chiral agent should have a twisting power high enough to induce the helical structure with a small amount of the chiral agent. The chiral agent should also have high compatibility with a liquid crystal compound, which enables effective interaction to occur between the components. Most of the conventional chiral agents of in the art are not sufficient with regard to these properties.
While Journal of Organic Chemistry, 56, 1991, 423 discloses bis-naphtol having a N-alkylphosphoric acid amide group, there are no cases reported on the use of bisnaphtol as a chiral agent for liquid crystal.
In recent years, much attention has been directed to hologram type optical recording media which are capable of recording information in a three-dimensional structure, as a recording medium which allows to write a large amount of information such as high-density image data.
The hologram type optical recording medium records information by superposing informing light which is modulated with a two-dimensional intensity distribution and reference light having intensity substantially equal to that of informing light within a photosensitive recording layer, and generating a distribution of optical property within the recording layer by using an interference pattern formed from informing light and reference light. To read (or reproduce) the written information, the recording layer is irradiated with only the reference light in a configuration similar to that of recording, so that such a reproduction light emerges from the recording layer which has intensity distribution corresponding to the distribution of optical property formed within the recording layer.
With the hologram type optical recording medium, since the distribution of optical property is formed in a three-dimensional configuration within the recording layer, multiplex recording is made possible in which a region where information is written by means of one informing light and another region where information is written by means of other informing light are partially superposed with each other. In case digital volume holography is employed, a very high signal-to-noise ratio (SN ratio) of one spot can be achieved, and therefore the original information can be reproduced with high fidelity, even when the SN ratio decreases somewhat due to the superposed recording. As a result, permissible number of multiplex recordings as large as several hundreds is made possible, thus resulting in a remarkable increase in the storage capacity of the optical recording medium (see JP-A No. 2002-123949).
The hologram type optical recording medium is exemplified by one shown in FIG. 1, which uses circularly polarized light for informing light and reference light and has such a constitution as a cholesteric liquid crystal layer or a dichroic mirror is disposed as a filter layer between a recording layer and a reflector film, and the recording layer and a servo layer are placed one on another (see JP-A No. 2004-265472).
The optical recording medium can be manufactured at a low cost in a large volume with high productivity, when the filter layer is constituted from a single-layer cholesteric liquid crystal layer which has helical structure of the same handedness as that of the circularly polarized light of the informing light. The filter layer constituted from this cholesteric liquid crystal layer has a sufficiently high reflectivity for writing light or reading light (wavelength 350 to 600 nm) which has circular polarization only, although the reflectivity decreases to as low as 20% when the recording system is changed to employ linearly polarized light or normal light, thus resulting in a larger proportion of light which leaks out.
To form a film compatible with the ordinary lens-based optics system by using the dichroic mirror, on the other hand, it is required to form 50 or more layers by vapor deposition which significantly increases the cost of manufacturing the optical recording medium.
As will be seen from the foregoing discussion, there has not been developed an efficient and low-cost method for mass production of hologram type optical recording medium which is capable of preventing random reflection of informing light and reference light on the reflector film of the optical recording medium, and preventing the occurrence of noises, and there is a pressing needs for such a method.